Security expert Bruce Schneier was been banned at the last minute from testifying in front of congress on the efficacy – or otherwise – of the US Transportation Security Administration's (TSA) much-maligned perv scanners.

Schneier is a long-time critic of the TSA's policies for screening travelers, and was formally invited to appear before the House Committee on Oversight and Government Reform and the Committee on Transportation and Infrastructure hearings. However, the TSA objected to his presence because he is currently involved in a legal case over the use of said scanners in US airports.

"I was looking forward to sitting next to a TSA person and challenging some of their statements. That would have been interesting," Schneier told The Register. "The request to appear came from the committee itself, because they'd been reading my stuff on this and thought it would be interesting."

Schneier, who is currently involved in an Economistdebate on just this issue, has criticized the TSA's procedures as "security theater", designed to give the appearance of security without actually being effective. He has pointed out that the scanners are easily defeated, and that since people who do have items are merely forced to give them up and sent on their way, terrorists simply need to send enough people through the systems until one of them succeeds.

This isn't the first time the TSA has been less than willing to have itself subject to anything like the same scrutiny that aircraft passengers are routinely put through. Last year they ducked out of similar hearings at the last minute, apparently because they didn't want to sit next to representatives from the Electronic Privacy Information Center (EPIC).

The use of the perv scanners is highly controversial. The TSA has spent millions of dollars to buy them, and the industry hired ex–Homeland Security supremo Michael Chertoff as a lobbyist to push the technology. However, there have been numerous examples of people claiming to be able to beat the scanners, concerns about the health implications of scanning, and the so-called "homosexual" pat-downs introduced to encourage people to use them caused a national day of protest.

There are currently several ongoing legal cases against the scanners, including one recent case in which, it is claimed, attractive female subjects were being repeatedly ordered to use the devices. Personal airport searches have to be performed by a member of the same sex as the target, but no such rules are in place for operators of the scanners.

"I think the TSA has really painted themselves into a corner over this," Schneier told us. "They've said the scanners were absolutely necessary for security, and made the pat downs you can have as an alternatives so unpleasant. It's going to be really hard for them to back down, if indeed they can."